1. Field of the Invention
The present invention relates to a vehicle electrical network having at least two energy storage devices, a method for operating such a vehicle electrical network, and means for implementing the method.
2. Description of the Related Art
The recovery of braking energy in motor vehicles through so-called recuperation is known. In a recuperation system used for this, during a braking process braking energy is converted into electrical energy by an electric machine, for example a claw pole generator, and is stored in a battery or in some other energy storage device. This energy is used to supply electric consumers, thus making it possible to save fuel.
If the mechanical braking power demanded by the driver is less than or equal to the performance capacity of the recuperation system, the braking typically takes place exclusively via the recuperation system. If the braking power requested by the driver is greater than this, then in addition a conventional (e.g. hydraulic) braking system is used for braking. In this case, it is not possible to convert all of the mechanical braking energy into electrical energy.
The possible saving of fuel through recuperation is therefore a function not only of the driving cycle and the driver's actions, but also of the performance capacity of the recuperation system. The performance capacity of the recuperation system is limited by the maximum generator power of the electric machine and by the maximum electric storage capacity of the energy storage device or devices in the vehicle electrical network.
In addition, so-called coasting operation is known as a fuel-saving measure. Here, during a so-called coasting phase the internal combustion engine is decoupled from the rest of the drive train. Through the decoupling of the internal combustion engine, the engine drag torque no longer acts on the rest of the drive train during the coasting phase. Therefore, the coasting phase of the vehicle is essentially lengthened, in comparison with so-called propulsion operation, when the internal combustion engine is coupled. If the internal combustion engine is switched off during the coasting phase, this results in a savings of fuel, as a function of the driving cycle and the driver's behavior. In vehicles having a conventional drivetrain, the electric consumers are supplied with power exclusively from the energy storage device or devices during the coasting phase, because the generator is no longer driven by the internal combustion engine.
Coasting and recuperation are not mutually exclusive, but rather can supplement one another. For example, coasting operation can be introduced when the driver is actuating neither the gas pedal nor the brake. The recuperation of braking energy can take place when the driver actuates the brakes.
The performance capacity for recuperation of 14V claw pole generators, standardly used in vehicles (passenger vehicles), having passive B6 bridge rectification is limited due to the low output voltage. It is true that the output voltage of claw pole generators can be increased as described in published German patent application document DE 197 33 221 A1, where a voltage adaptation to the 14V vehicle electrical network is carried out using a DC/DC converter. However, the use of a DC/DC converter increases costs. Known vehicle electrical networks also have further problems, and are frequently not effectively usable, or not sufficiently so, for carrying out both recuperation and coasting operation, as is explained below.
Therefore, there is a need for vehicle electrical networks that can be used flexibly for recuperation and coasting operation, which in particular also can provide adequate recuperation power and are fail-safe.